The present invention relates to methods and apparatuses for damping vibration, more particularly for attenuating acoustic vibration such as structureborne noise.
Vibrations can cause problems such as structural weakening, metal fatigue and bothersome noise. Of particular note are situations wherein a power-driven source (e.g., a motor) produces a frequency at which an attached structure naturally vibrates, a phenomenon known as xe2x80x9cresonance.xe2x80x9d
Various types of passive damping treatments have been known to be effective in reducing the amplitude of vibrations at resonant frequencies. Among the known effective passive damping methodologies are freelayer damping, constrained layer damping, tuned damping and direct-load-path damping.
According to freelayer damping, damping material is positioned on the to-be-damped structure. When the structure is excited, the damping material and the structure vibrate together. This causes the damping material to stretch and compress, resulting in the dissipation of vibration energy as heat
According to constrained layer damping, damping material is positioned against the structure and covered with a constraining layer, such as a metal sheet In other words, the damping material is sandwiched between the constraining layer and the structure. The mechanism between the constraining layer and the structure is shear in nature.
According to tuned damping, damping material acts as a spring-mass system which is tuned to vibrate at a frequency of interest, e.g., the same frequency as the structural vibration of the structure on which it is mounted. The tuned damper vibrates out-of-phase with the structure and applies a force opposite the motion of the structure.
According to direct-load-path damping, a discrete location of the structure is damped by load-bearing damping material. In contrast, freelayer damping and constrained layer damping afford area coverage of the structure.
Generally, tuned damping provides narrowband damping, designed to concentrate damping where most needed, i.e., at the frequencies of resonance modes. By comparison, freelayer damping, constrained layer damping and direct-load-path damping provide broadband damping, designed to afford more moderate damping over a wider range of frequencies.
The United States Navy has recently begun evaluating new mechanisms in association with a family of damping treatments, viz., entrained damping, which utilizes particulate material. According to entrained damping, a hollow space in a structure to be damped is filled with particulate damping material. Particles such as sand or polymeric (e.g., viscoelastic) beads have been demonstrated by the U.S. Navy to provide effective structureborne noise attenuation when used as a filler for noise-critical structures.
In particular, the U.S. Navy has experimentally found that this particle-filler type of damping treatment can be adapted to effectuate a kind of tuned damping; that is, the particle filler can act as a tuned absorber, wherein the vibration of the structure couples into the particle filler. At the fundamental frequencies, where the structural vibration coincides with the standing wave resonance modes through the thickness of the particle filler, very high levels of structural damping have been achieved in proprietary testing conducted by the U.S. Navy.
The mechanisms associated with the above-described xe2x80x9cparticle-typexe2x80x9d tuned damping are not clearly understood; nevertheless, considered likely by U.S. Navy researchers are several mechanisms which influence the high levels of damping obtained.
One mechanism associated with tuned-particle-type damping is believed to be the relative rigid body displacement between the particles of the filler, which adds a frictional or coulomb-type damping to the structure. This mechanism is likely the dominant one for a structure filled with sand; the sand particles, made of quartz or other mineral base material, are very stiff with extremely low levels of inherent material damping.
Another mechanism associated with tuned-particle-type damping is believed to be present for viscoelastic-type filler materials. This effect also occurs at the interface of the individual particles. When an acoustic wave is transmitted through a solid material, the pressure is relatively evenly distributed through the material across any section whose dimensions are small relative to the wavelength. In the case of a medium made up of particles, however, the point contact between each particle forces a pressure concentration and subsequent strain-increase at each particle interface as the pressure wave is transmitted. The strain concentration at each contact point has the effect of multiplying the viscoelastic effect of the base material over an equivalent solid material.
Hence, the U.S. Navy has been evaluating, with good success, the application of free viscoelastic particle damping treatments to entraining structures (i.e., structures which can be filled). The U.S. Navy has especially been assessing the application of free viscoelastic particle damping treatments of a tuned-particle-type to entraining structures. The U.S. Navy is currently desirous of effectuating, with respect to non-entraining structures (i.e., structures which cannot be filled, such as plates, or piping having working fluid inside), damping treatments having attributes of both entrained damping and tuned damping.
In view of the foregoing, it is an object of the present invention to provide method and apparatus for effectuating, with respect to a non-entraining structure, a damping treatment which is similarly effective as is a tuned-particle-type damping treatment with respect to an entraining structure.
It is a further object of the present invention to provide such method and apparatus which effectuate same in an efficient and effective manner.
In accordance with many embodiments of the present invention, a passive vibration damping device comprises plural viscoelastic particles which generally cohere with one another. At least substantially every said particle is characterized by: (a) adherent communication with at least one other particle; (b) adjacency to at least one separation between the particle and at least one other particle; and, (c) at least one of: (i) an approximate shear modulus in the range between about 10 p.s.i. and about 100,000 p.s.i. inclusive; and, (ii) an approximate loss factor in the range between about 0.05 and about 1.5 inclusive. According to typical practice, the adherent communication includes a contact region between two particles, and a bond region between the two particles, whereing the bond region is at least approximately commensurate with the contact region. Frequent inventive practice prescribes particle sizes wherein at least substantially every particle is characterized by size (as taken along its greatest dimension) in the range between about 0.05 inches and about 0.5 inches inclusive.
Further provided by the present invention is a method for effectuating passive damping of an object The method comprises: (a) providing a device including plural viscoelastic particles which generally cohere with one another; and (b) coupling the device with the object. At least substantially every particle is characterized by: (a) adherent communication with at least one other particle; (b) adjacency to at least one separation between the particle and at least one other particle; and, (c) at least one of: an approximate shear modulus in the range between about 10 p.s.i. and about 100,000 p.s.i. inclusive; and an approximate loss factor in the range between about 0.05 and about 1.5 inclusive. Typically according to inventive practice, at least substantially every said particle is characterized by size in the range between about 0.05 inches and about 0.5 inches inclusive.
Further provided by the present invention is a method for making a device suitable for passively damping a structure. The method comprises: (a) providing plural viscoelastic particles; (b) placing the particles in a mold; and (c) causing the particles to generally cohere with one another. At least substantially every particle is characterized by: (a) adherent communication with at least one other particle; (b) adjacency to at least one separation between the particle and at least one other particle; and, (c) at least one of: an approximate shear modulus in the range between about 10 p.s.i. and about 100,000 p.s.i. inclusive; and an approximate loss factor in the range between about 0.05 and about 1.5 inclusive. Typically according to inventive practice, at least substantially every said particle is characterized by size in the range between about 0.05 inches and about 0.5 inches inclusive.
The xe2x80x9cshear modulusxe2x80x9d (also known as the xe2x80x9cmodulus of elasticity in shear,xe2x80x9d xe2x80x9cmodulus of rigidityxe2x80x9d or xe2x80x9ccoefficient of rigidity) is a measure of the resistance of a material to shearing stress. The shear modulus equals the shearing stress divided by the resultant angle of deformation (typically expressed in radians). The xe2x80x9closs factorxe2x80x9d is the specific damping capacity per radian of the damping cycle. Damping can be defined in terms of energy dissipation and the peak potential energy as xcex7=xcex94U/2xcfx80Umax, where xcex7 is the loss factor, Umax is the peak potential energy, and xcex94U is the energy dissipated (e.g., change in potential energy). The loss factor, xcex7, is equal to the ratio between the energy dissipated per radian, xcex94U/2xcfx80, and the peak potential energy, Umax.
The present invention provides a methodology for efficiently and effectively practicing tuned-particle-type damping treatment with respect to non-entraining structures. Inventively featured is the adhering (e.g., sintering) of viscoelastic particles to each other so as to form a continuous, self-supporting, easily attachable matrix of such particles. In particular, a key feature of this invention is the development of self-supporting bonds between the particles that make up the body of the damping material; a bond is developed at each of the contact points between the particles. Advantageously, the inventive matrix of viscoelastic particles is easily affixable to non-entraining structures.
The application of particles or a xe2x80x9cparticulate qualityxe2x80x9d to non-entraining structures represents a difficult practical challenge which, if not overcome, would limit such application. The present invention solves this problem.
According to typical embodiments of this invention, sintered particle material is used as a damped tuned vibration absorber for structural noise attenuation. The damping material comprises a matrix (array or arrangement) of plural, approximately spherical viscoelastic particles (also appropriately described as xe2x80x9cpelletsxe2x80x9d, xe2x80x9cbeads,xe2x80x9d xe2x80x9cspheresxe2x80x9d or xe2x80x9cmicrospheresxe2x80x9d) which slightly and fixedly contact one another. The particles, at least approximately spherical, are set so as to minimally touch, but not impinge upon, each other. When thus disposed, the particles esssentially or substantially retain their original spherical character, at the same time constituting a unified whole.
Inventive practice does not necessarily prescribe a spherical or near-spherical shape of the viscoelastic particles, or that they be uniformly sized, or that they be made of the same material. Varieties of particle sizes, particle shapes (spherical or otherwise) and particle material compositions are inventively practicable. According to diverse inventive embodiments, the viscoelastic particles can vary in size and/or shape and/or material. In this regard, regardless of the combinations and configurations of particles, it is inventively required that the viscoelastic particles be fixedly contiguous with respect to each other, but that they not be entirely contiguous (i.e., that they be xe2x80x9cslightly,xe2x80x9d xe2x80x9cpartially,xe2x80x9d xe2x80x9cmoderately,xe2x80x9d xe2x80x9cdiscretely,xe2x80x9d xe2x80x9cnoncontinuouslyxe2x80x9d or xe2x80x9cnonsolidlyxe2x80x9d contiguous) with respect to each other; in other words, the particles define spaces, gaps or interstices therebetween which remain in the matrix.
The viscoelastic particles are adhered to each other in a manner which maintains the pressure and strain concentration points while not allowing individual particles to be released from the matrix. According to many inventive embodiments, a structureborne noise attenuating treatment comprises viscoelastic particles which have been fused together to provide a coherent mass which is self-supporting.
Other objects, advantages and features of this invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.
The following appendices are hereby made a part of this disclosure:
Attached hereto marked xe2x80x9cAPPENDIX Axe2x80x9d and incorporated herein by reference is the following 63-page report (2 title pages, pages i-v and pages 1-56): Geoffrey J. Frank, Michael L. Drake and Steven E. Olson, xe2x80x9cModeling of Damping in Tubes Using Viscoelastic Beads,xe2x80x9d Aerospace Mechanics Division, University of Dayton Research Institute (UDRI), Dayton, Ohio 45469-0110, Final Technical Report, December 1995, UDR-TR-95-79. This is a private, proprietary final report which was prepared for the Naval Surface Warfare Center, Carderock Division, Annapolis, Md., 21402, pursuant to Contract No. N00167-94-C-0097.